Next generation Ethernet is likely to have a data rate around 100 Gb/s. One possibility to accomplish 100-Gb/s transmission is the use of a multiplex of parallel lower-speed channels. However, the parallel approach typically has a low spectral efficiency, requires temporal de-skewing among channels, and has a large footprint or consumes significant chip real-estate. Another possibility is the use of a single 100-Gb/s serial channel. With a multi-level modulation format, such as differential quadrature phase-shift keying (DQPSK), in which the data is encoded using four different phase levels, the serial approach can have a high spectral efficiency, and there is no need for de-skewing. Many existing 10-Gb/s systems use pluggable transceivers. It would be highly desirable to make a 100-Gb/s multi-level modulator that is small enough to fit in a pluggable transceiver.
DQPSK modulators demonstrated to date have been too large for a pluggable transceiver because they employ phase modulators based on GaAs or LiNbO3. A reported GaAs DQPSK modulator was 52 mm long, R. Griffin, R. Johnstone, R. Walker, S. Wadsworth, A. Carter, and M. Wale, “Integrated DQPSK transmitter for dispersion-tolerant and dispersion-managed DWDM transmission,” Optical Fiber Communication Conference, paper FP6, 2003, and a reported LiNbO3 DQPSK modulator was more than 43 mm long, K. Higuma, S. Mori, T. Kawanishi, and M. Izutsu, “A bias condition monitor technique for the nested Mach-Zehnder modulator,” IEICE Electronics Express, vol. 3, pp. 238-242, 2006. These modulators use a traditional DQPSK modulator design consisting of a nested pair of Mach-Zehnder modulators.
The existing modulators are so long because of the relatively weak electro-optic effect in GaAs and LiNbO3. A solution is to make this design in InP, which has a much stronger electro-optic effect in the C-band by using the quantum-confined Stark effect. However, despite an increased electro-refractive effect, a phase shifter in InP with a reasonable Vπ is still quite long, 0.5, L. Zhang, J. Sinsky, D. Van Thourhout, N. Sauer, L. Stulz, A. Adamiecki, and S. Chandrasekhar, “Low-voltage high-speed traveling wave InGaAsP-InP phase modulator,” IEEE Photon. Technol. Lett., vol. 16, pp. 1831-1833, August 2004 to 4 mm, H. N. Klein, H. Chen, D. Hoffmann, S. Staroske, A. G. Steffan, and K.-O. Velthaus, “1.55 μm Mach-Zehnder modulators on InP for optical 40/80 Gbit/s transmission networks,” Integrated Photonics Research M, paper TuA2.4, 2006, and so requires a traveling-wave structure. A traveling-wave structure in InP is highly demanding to fabricate.
Thus there is a need for a new approach to making a DQPSK modulator.